To get a stable doping profile induced by proton irradiation one has to go for very oxygen lean floating zone (FZ) silicon material but at the cost of limited wafer diameter, since the production of floating zone silicon is not feasible for 300 mm diameter wafers and above which are required for various applications. Hence, there is a particular interest for silicon wafers with a diameter of 300 mm and more. Wafers with such large diameters can be manufactured from silicon ingots grown by the Chzochralski method, in particular by the magnetic Czochralski (MCz) method.
In MCz silicon, carbon and oxygen are the most abundant impurities. In the case of proton doping, the final doping concentration is critically affected by the presence of carbon atoms that are predominantly positioned in substitutional lattice sites.
In the following description, substitutional impurities, i.e., impurities located in substitutional lattice sites, will be labelled by a subscripted “S”. Interstitial impurities will be labelled by a subscriped “I”.
The carbon content in silicon is typically in the range of 5·1014-1·1016 cm−3 which is enough to critically influence the proton-induced doping profiles. Therefore, an accurate measurement of the carbon content in this range is necessary in order to control the final proton-induced doping concentration.
Conventionally, the carbon content in silicon is determined by SIMS (Secondary Ion Mass Spectroscopy) or FTIR (Fourier Transform Infrared Spectroscopy). The utilization of these methods, however, is limited due to their rather high detection limits for carbon contents of about 3·1015 cm−3.
The sensitivity of FTIR is strongly impaired by the interference of the carbon (CS) and silicon (SiS) signals having nearly the same resonant frequencies or by the necessity of using reference samples with known low carbon contents which are not available as industry standards.
In addition, the conventional FTIR method can only detect substitutional carbon atoms CS located in regular lattice sites. Interstitial carbon atoms CI originating from other high temperature or irradiation process steps and possibly making up a substantial fraction of the carbon present in a silicon sample, however, are not detectable by this method. This also limits the sensitivity of the conventional FTIR method.